HELP! PL400 crackling/static

[Lewcifer]

Ugh, thanks for your patience.

 

[laatsch55]

Call me Job...

 

[Lewcifer]

After hooking your meter to the bias resistor, turn the amp on and see what it is. Offset has to be checked with amp on..

Okay, at those resistors, I'm seeing 507mv on the first one and 476mv on the third one.
For the outputs, I'm getting .2mv where the left channel enters the relay, and 16.5 for the right side.
those readings at the output are pretty much the same as they were before I started playing with this thing.

 

[laatsch55]

Ok....have you checked rail voltages yet? Minus lead on the bus between the caps, positive on the plus and then minus cap terminals...should be about 80 volts DC..

 

[laatsch55]

The 16.5 reading ....is that volts or Mv?

 

[Lewcifer]

Ok....have you checked rail voltages yet? Minus lead on the bus between the caps, positive on the plus and then minus cap terminals...should be about 80 volts DC..

Just a couple questions to clarify because I want to do this safely and not get my 1980s hairstyle back.
the copper bar is the bus?
Positive terminals are the ones closest to the back wall?
So I just touch my negative lead to the copper bar and positive lead to the terminal at the back?
Apologies for all the beginner questions. I really appreciate your help.

 

[Lewcifer]

The 16.5 reading ....is that volts or Mv?

MV

 

[laatsch55]

Ok, the copper bus bar is the ground. The other terminal of each cap is your plus and minus rail voltage , should have plus and minus 80 vdc...

 

[Lewcifer]

Ok, the copper bus bar is the ground. The other terminal of each cap is your plus and minus rail voltage , should have plus and minus 80 vdc...

Thanks! I’m getting about 78V on both.

 

[laatsch55]

Your getting a minus on the bottom cap aren't you?? The one with the green wire?

 

[Lewcifer]

Your getting a minus on the bottom cap aren't you?? The one with the green wire?

Oh, yes! about 78 on the upper one and -78 on the lower one.
Just checked it again.

 

[laatsch55]

Well nothing terrible is wrong, are you getting a relay click on the DCP on power up?

 

[Lewcifer]

Well nothing terrible is wrong, are you getting a relay click on the DCP on power up?

Yeah, it does click every time I plug it in.

 

[laatsch55]

Ok, that's a good sign. Have you read up on adjusting bias? That's next, bring bias down to 375 mv.....

 

[Lewcifer]

Ok, that's a good sign. Have you read up on adjusting bias? That's next, bring bias down to 375 mv.....

I did print out a service manual today. I will look into it. Thank you SO much.

 

[laatsch55]

Hook your meter to the first row 10 ohm bias resistor, turn amp on and adjust thumbwheel pot while observing reading. With those old open pots it can get real erratic..

 

[Lewcifer]

Hook your meter to the first row 10 ohm bias resistor, turn amp on and adjust thumbwheel pot while observing reading. With those old open pots it can get real erratic..

Seems like I should invest in some alligator clips before proceeding. Or I might be able to borrow some from the mechanic at work.

 

[laatsch55]

Or some "mini grabbers"....google that...

 

[laatsch55]

Pomona mini grabbers..

 

[Lewcifer]

Ooh, perfect. Just ordered a set. Looks like the adjustments are going to be R20 on the driver board when I get back into it? I see two Of them marked R20 on the diagram.